High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study

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<ul><li><p>sal S</p><p>ero 1taledrobnhib, reon</p><p> 2011 Elsevier Ltd. All rights reserved.</p><p>ated inease cn conupgrafuentudge (0% of i</p><p>WWTPs in some undeveloped countries. For example, up to 2010,among the 2500WWTPs in China, only 50 ones were designed withanaerobic digestion systems, and only 20% of them were well oper-ated. Low-solid anaerobic digestion was not well applied in Chinamainly due to poor management, unprofessional operation,economical limitation and inadequate planning. However, morethan 80% of the sewage sludge in China has already been dewatered</p><p>shima et al. (1999) has suggested a system in which the dewateredsludge discharged from small-scale plants is collected and sent to aplant with an anaerobic digester. They investigated the effect ofmoisture content on anaerobic digestion of dewatered sludge, butthe TS contents of the fed sludge used in their study were below11% (w/w), and a long-term effect was not investigated. Nges andLiu, 2010 investigated the effect of solid retention time (SRT) onanaerobic digestion of dewatered sludge inmesophilic and thermo-philic conditions in long-term experiments, but the TS content offed sludge was below 12%. Up to now, there are few studies related</p><p> Corresponding author. Tel.: +86 21 65981794; fax: +86 21 65983602.</p><p>Bioresource Technology 104 (2012) 150156</p><p>Contents lists available at</p><p>T</p><p>elsE-mail address: tj_dongbin@163.com (B. Dong).essary stabilization. Thus, it is essential to develop proper treat-ment processes to reduce the amount of sludge. One of the mostwidely used processes is anaerobic digestion or methanisation. Itcan realize sludge stabilization by converting a part of its organicmatter into biogas which is a renewable energy source. This tech-nology has been successfully implemented in the treatment ofagricultural wastes, food wastes, and sewage sludge (Chen et al.,2008).</p><p>However, the use of traditional anaerobic digester for low-solidsewage sludge is not always feasible in small-scale WWTPs or</p><p>less material handling, and so on (Guendouz et al., 2008).So far, a wide range of organic solids found in municipal, indus-</p><p>trial, and agricultural wastes have been investigated as feedstocksin high-solid anaerobic digestion, including food wastes (Choet al., 1995; Lu et al., 2007), agricultural wastes (He et al., 2008; Lis-sens et al., 2004; Mosier et al., 2005; Pang et al., 2008), and organicfraction ofmunicipal solidwastes (OFMSW) (Bolzonella et al., 2006;Forster-Carneiro et al., 2007; Martin et al., 2003; Sans et al., 1995).However, so far, no reports can be found focusing on high-solidanaerobic digestion of sewage sludge with feeding TS of 20%. Fuji-1. Introduction</p><p>Volume and mass of sludge generplants (WWTPs) are expected to incrdecade, due to increasing populatioworks, building of new WWTPs andto fulll the more stringent local ethe annual production of sewage slhas reached almost 3000 tons, and 80960-8524/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.biortech.2011.10.090waste water treatmentontinuously in the nextnected to sewage net-ding of existing plantsregulations. In China,80% moisture content)t has not obtained nec-</p><p>before further disposal or treatment, whichmakes it favorable to becentralized processed. Hence, high-solid anaerobic digestion couldbe one viable option to solve the sludge disposal problems inundeveloped countries like China or in small-scale WWTPs.High-solid anaerobic digestion is usually characterized by a highTS content of the feedstocks, typically greater than 15% (w/w)(Rapport et al., 2008) and has been claimed to be advantageous overtraditional low-solid anaerobic digestion for several reasons, suchas smaller reactor volume, lower energy requirements for heating,High-solidMesophilic</p><p>higher OLR (46 times as high) and obtain similar methane yield and VS reduction as conventionallow-solid system at the same SRT, thus reach much higher volumetric methane production rate.High-solid anaerobic digestion of sewageFeasibility study</p><p>Nina Duan, Bin Dong , Bing Wu, Xiaohu DaiNational Engineering Research Center for Urban Pollution Control, School of Environment</p><p>a r t i c l e i n f o</p><p>Article history:Received 10 September 2011Received in revised form 22 October 2011Accepted 25 October 2011Available online 3 November 2011</p><p>Keywords:Anaerobic digestionSewage sludge</p><p>a b s t r a c t</p><p>Feasibility of high-solid anastirred tank reactors at 35retention time (SRT) and toExperimental results show600 mg l1, high-solid anaemoderate and signicant i600 and 600 to 800 mg l1</p><p>nicant ammonia inhibiti</p><p>Bioresource</p><p>journal homepage: www.ll rights reserved.ludge under mesophilic conditions:</p><p>cience and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, PR China</p><p>bic digestion of sewage sludge was investigated in single-stage completelyC. System stability and the effect of organic loading rate (OLR), sludgesolid (TS) content on the performance of high-solid system was examined.that, with the concentration of free ammonia nitrogen (FAN) lower thanic digestion of sewage sludge could maintain satisfactory stability. Slight,ition was found with FAN concentration ranging from 250 to 400, 400 tospectively. The VFA/TA criteria could not foresee system instability in sig-system by its traditional ratio grades. High-solid system could support</p><p>SciVerse ScienceDirect</p><p>echnology</p><p>evier .com/locate /bior tech</p></li><li><p>to high-solid anaerobic digestion of sewage sludge with TS above12% in a long-term operation. In this study, high-solid mesophilicdigestion of sewage sludge (feeding TS 20%) was investigated usingsingle-stage completely mixed reactors (semi-continuously oper-ated), the performance of which were compared with reactors fedby TS of 10% and 15%. The effect of OLR, SRT and TS content wasexamined, with special attention paid to ammonia inhibition andsystem stability in high-solid state. The aim was to investigate</p><p>6890N, CA, USA) with a thermal conductivity detector equipped</p><p>10,000 rpm for 10 min. Then the supernatant was passed througha microber lter (0.45 lm) and the ltrate was acidied by formicacid to adjust the pH to approximately 2.0 before VFA was ana-lyzed by a GC (Agilent Technologies 6890N, CA, USA) with ameionization detector. TS, VS, TA and TAN were determined accordingto standard methods (APHA, 1995). FAN concentration was calcu-lated in the same way as described by stergaard (1985). The deg-radation or removal level based on VS (i.e. VS reduction) was</p><p>tion), indicating better buffering capacity and successful recovery</p><p>es of</p><p>s 11</p><p>N. Duan et al. / Bioresource Technology 104 (2012) 150156 151with Hayseq Q mesh and Molsieve 5A columns. To analyze VFAs,the sludge samples from the reactors were centrifuged at</p><p>Table 1Characteristics of the substrate and inoculums (TS and VS values reported are averag</p><p>Parameters DSa 1 (days 155) DS 2 (days 56115) DS 3 (day</p><p>pH 7.4 7.4 7.5TS (%, w/w) 20.45 22.56 23.08VS/TS (%) 60.09 60.29 52.48TAN (mg l1) 783 694 801the stability of high-solid anaerobic digestion of sewage sludgeand to evaluate its performance.</p><p>2. Methods</p><p>2.1. Substrates and inoculums</p><p>Dewatered sewage sludge from Anting WWTP (Shanghai, Chi-na) was used as substrate for the present study. The sludge was ob-tained by collecting primary and excess sludge and dewatered withthe aid of a high-molecular occulant based on polyacrylamide.The total solids (TS) of the dewatered sludge ranged from 19% to23% (w/w) and volatile solids (VS) accounted for 5060% of TS.The mesophilic seed sludge collected from an anaerobic digesterat Bailonggang WWTP (Shanghai, China) had TS of 2.3% (w/w)and VS was 56.5% of TS. Characteristics of dewatered sludge andinoculums are listed in Table 1. The collected sludge was storedat 4 C and heated to 35 C before everyday feeding.</p><p>2.2. Reactors and operation</p><p>Three identical reactors (numbered R1, R2 and R3), with liquidworking volume of 6.0 l, were equipped with helix-type stirrers,which were set at a rate of 60 rpm (rotations per minute) with10 min stirring and 10 min break continuously. Volumes of pro-duced biogas were measured by wet gas meters every day.</p><p>On the rst day of the experiments, 6.0 l seed sludge was addedto each reactor, which was operated semi-continuously (once-a-day draw-off and feeding) at 35 1 C. During the start-up period,when the TS concentration of the substrate in the reactor did notreach its designed level, the OLR was increased stepwise withdewatered sludge as feedstock. Once the TS of the substrate in eachreactor approached its designed level, the feeding sludge was di-luted to its designed TS level (10%, 15% and 20%, respectively) withde-ionized water before feeding.</p><p>2.3. Analytical methods</p><p>Biogas and substrate samples of the reactors were taken twice aweek and analyzed for methane content, pH, TS, VS, volatile fattyacid (VFA), total alkalinity (TA), total ammonianitrogen (TAN)and free ammonianitrogen (FAN). Methane content of the biogaswas measured by a gas chromatograph (GC) (Agilent TechnologiesFAN (mg l1) 26 23 33</p><p>a Dewatered sludge.of the systems.It should be noticed that, although the system was inoculated</p><p>by digestate from traditional low-solid digester treating sewagesludge and experienced intense unstable state (VFA/TA 0.8-1.1)by overloading, it could still successfully recover in continuousoperation at OLR 2.0 kg VS m3 d3, which was still higher thanthe regular OLR applied in low-solid digesters treating sewagesludge. The results indicated that, as long as pH was not dramati-cally inuenced and was kept in favorable range for methanogens,previous high VFA/TA state was quite recoverable in continuous</p><p>three measurements).</p><p>6141) DS 4 (days 142160) DS 5 (days 161200) Inoculums</p><p>7.4 7.6 7.819.72 22.35 2.3152.47 51.17 56.51</p><p>752 739 321calculated by the same equation as reported by Koch et al.(2009), assuming that the mass of undegradable material (inor-ganic fraction) is constant:</p><p>VSreduction 1 VSdigestate 1 VSfeed VSfeed 1 VSdigestate %; 1</p><p>where VSdigestate is the loss on ignition of digestate (% of TS) andVSfeed is the loss on ignition of feeding sludge (% of TS).</p><p>3. Results and discussion</p><p>3.1. Start-up performances</p><p>Figs. 13 show the performance data of R1, R2 and R3, whichwere fed with dewatered sludge until each reached its designedTS level (on day 30, 68 and 85, respectively). Then the feedingsludge was diluted to its designed TS level (10%, 15% and 20%,respectively) with de-ionized water before feeding. In the rst17 days, each reactor was operated at an OLR of 4.1 kg VS m3 d3,which was much higher than that in traditional low-solid digestionsystem for sewage sludge (Metcalf and Eddy, 2003; Qasim, 1999;Turovskiy and Mathai, 2006). Signicant VFA accumulation andsharp increases of VFA/TA ratio were observed, accompanied bydecreasing biogas production (days 520) and VS reduction (days1220), which indicated low stability at high starting OLR, proba-bly due to the small amount of biomass and its poor acclimationto the substrate at the beginning.</p><p>As OLR reducing to 2.0 kg VS m3 d3, methane yield of eachreactor showed increasing trend with great uctuation, probablydue to the conversion of accumulative VFAs to biogas. From day20 to the end of the start-up period (day 30, 68 and 85, respec-tively), the TA, VS reduction and TAN concentration in each reactorshowed increasing trend with TS increasing (the drop of TAN con-centration on day 20 may be caused by the initial drop of VS reduc-25 38 26</p></li><li><p>echn0.0</p><p>2.5</p><p>5.0</p><p>7.5</p><p>10.0</p><p>12.5</p><p>15.0 OLR SRT TS</p><p>OLR</p><p> ( kg</p><p> VS </p><p>/ m3 .</p><p>d )</p><p>200</p><p>300</p><p>400</p><p>500</p><p>600</p><p>FAN</p><p> (mg/</p><p>l)</p><p>152 N. Duan et al. / Bioresource Toperation, because VFA accumulation would help providing suf-cient substrate for multiplication of methanogens to same extent.</p><p>3.2. Ammonia inhibition and system stability</p><p>It should be noticed that, at the end of the start-up period of R2,the VFA concentration was still relatively higher than normal level.At the end of the start-up period of R3, the VFA concentrationraised rapidly to 4500 mg l1 at day 85, with a dramatic decreaseof methane yield. That may probably due to the decreasing meth-anogenic activity caused by ammonia inhibition (De Baere et al.,1984; Hansen et al., 1998; Lay et al., 1997). High VFA levels and al-most steady VS reduction in this period indicated that the acido-genic activity was not inuenced signicantly, which was inaccordance with the report of Koster and Lettinga (1988).</p><p>To relieve the dual inhibition caused by ammonia and high VFA,the OLR of Reactor 3 was reduced to 2.0 kg VS m3 d3 after day 85.Since free ammonia has been suggested to be the actual toxicagent, the concentrations of both TAN and FAN are shown in Figs.13.</p><p>It was obvious that methanogens were not evidently inuencedwith TAN 10002500 mg l1 and FAN lower than 250 mg l1</p><p>(Fig. 1), since no VFA accumulation and deterioration in biogasproduction (the sharp decrease on day 116 was due to the VS/TSchange in feeding sludge) was shown. Evident inuence was</p><p>0</p><p>100</p><p>0 20 40 60 80 1000</p><p>5</p><p>10</p><p>15</p><p>20</p><p>25</p><p>30</p><p>35</p><p>40</p><p>45</p><p>VS re</p><p>duct</p><p>ion </p><p>(%)</p><p>time (d</p><p>Fig. 1. Performance data of0</p><p>10</p><p>20</p><p>30</p><p>40</p><p>50</p><p>60</p><p>SRT </p><p>(d), </p><p> TS </p><p>(w/w</p><p> %)</p><p> FAN VFA TA TAN pH</p><p>4</p><p>6</p><p>8</p><p>10</p><p>12</p><p>VFA</p><p>,TA</p><p>,TA</p><p>N (g</p><p>/l); p</p><p>H</p><p>ology 104 (2012) 150156exhibited at FAN 500600 mg l1 (Figs. 2 and 3), which led to slightaccumulation of VFAs. Signicant inhibition occurred with FAN600800 mg l1 (Fig. 3), leading to intense VFA accumulation anddramatic drop of biogas production.</p><p>The results suggested that, in high-solid anaerobic digestion ofsewage sludge, free ammonia concentration was an important fac-tor inuencing the stability of the system. Taken into considerationof VFA accumulation and biogas production as the main factors toevaluate the real stability degree (VFA accumulation without dra-matic drop of biogas production was considered steady state, anddramatic drop of biogas production was considered a sign of greatinstability), the inhibitory effect of free ammonia can be classiedto several degrees as listed in Table 2.</p><p>As shown in Table 2, in a high-solid anaerobic system, whereboth free ammonia and VFAwere important factors affecting meth-anogenic activity, the parameter of VFA/TA ratio was not feasible toevaluate the stability of the system. In the situation of slight inhi-bition, system stability was affected mainly by VFA concentration,and successful recoverywas proved at high OLR (2.0 kg VS m3 d3)with VFA accumulation up to 10 g l1 (see start-up performance)when the VFA/TA ratio was near 1.0. Once full recovery was ob-tained from overloading, the VFA/TA ratio would decrease to0.030.12 in the slight inhibition situation. In the situation ofmoderate inhibition, slight VFA accumulation existed as a resultof decreased methanogenic activity caused by ammonia inhibition,</p><p>0</p><p>2</p><p>120 140 160 180 200</p><p> VS reduction methane yield (Y) VFA/TA</p><p>0.0</p><p>0.1</p><p>0.2</p><p>0.3</p><p>0.4</p><p>0.5</p><p>0.6</p><p>0.7</p><p>0.8</p><p>0.9</p><p>)Y </p><p>( l / </p><p>g VS</p><p>adde</p><p>d ), V</p><p>FA / </p><p>TA</p><p>R1 (feeding TS of 10%).</p></li><li><p> O S T</p><p> TAN pH 12</p><p>14</p><p>/l); p</p><p>H</p><p>echn0.0</p><p>1.5</p><p>3.0</p><p>4.5</p><p>6.0</p><p>7.5</p><p>9.0</p><p>OLR</p><p> ( kg</p><p> VS </p><p>/ m3 .</p><p>d )</p><p>200</p><p>300</p><p>400</p><p>500</p><p>600</p><p>700</p><p>800</p><p>900</p><p>FAN</p><p> (mg/</p><p>l)</p><p>N. Duan et al. / Bioresource Tb...</p></li></ul>